Hermetically sealed semiconductor device with corrosion inhibited ferrous metal portions

ABSTRACT

Semiconductor devices hermetically sealed within metal cases formed, at least in part, of laminated or clad ferrous metal are subject to corrosion of the exposed portions of the ferrous metal. The invention inhibits the corrosion of the ferrous metal by providing a phosphatized ferrous metal surface which is coated with a transparent, hydrophobic, organic flux agent having a melting point above 70* C such as, for example, a stearic acidaluminum stearate mixture.

Umted States Patent 1191 [111- 3,831,066

Gabrail Aug. 20, 1974 HERMETICALLY SEALED 2,448,397 8/1948 Schilling et al. 148/6.l5 SEMICONDUCTOR DEVICE WITH 2,875,110 2/1959 Rossaciinder l48/6.15

, 2,930,723 3/1960 Drys ale etal 148/6.15

CORROSION INHIBITED FERROUS METAL 3,397,077 8/1968 Boller et al. l48/6.l5

PORTIONS Inventor: Sami Ibrahim Gabrail, Syracuse,

[75] N Y Primary Examiner-Rudolph V. Rolinec Assistant Examiner-E. Wojciechowicz [73] Assignee: General Electric Company, Attorney, Agent, or Firm-Robert J. Mooney Syracuse, N.Y.

[22] Filed: Apr. 10, 1972 ABSTRACT [21] Appl. No.: 242,813

Semiconductor devices hermetically sealed within Related Apphcamn Data metal cases formed, at least in part, of laminated or Continuation of 44,127, June 1970, clad ferrous metal are subject to corrosion of the exabandonedposed portions of the ferrous metal. The invention inhibits the corrosion of the ferrous metal by providing [52] 317/234 3 171/234 148/615 a phosphatized ferrous metal surface which is coated [51] Int. Cl. H01] 5/00 with a transparent,hydrophobic, Organic flux agent [58] Field of Search 317/234 G having a melting point above 70 C Such as, for exam ple, a stearic acid-aluminum stearate mixture. [56] References Cited UNITED STATES PATENTS 6 Claims, 2 Drawing Figures 2,104,637 1/1938 Crooks 91/68 I 2 I 2 A Lx\\ 3 K 32 A OJ 2 -L 26- PAIENIEUMIBZOIQH I 3.831 065 FIG.2.

BOND METAL CASE T0 BASE 0F SEMICONDUCTOR DEVICE TO FORM HERMETIC SEAL.

TREAT EXPOSED FERROU$ METAL PORTIONS 0F METAL CASE WITH PHOSPHATE SOLUTION.

COAT PHOSPHAT/ZED METAL WI TH TRANSPARENT, HYDROPHOB/C, ORGANIC, FLUX AGENT.

INVENTORZ SAMI I. GABRAIL BY W/ HIS ATTORNEY.

HERMETICALLY SEALED SEMICONDUCTOR DEVICE WITH CORROSION INHIBITED FERROUS METAL PORTIONS This is a continuation, of application Ser. No. 44,127, filed June 8, 1970, now abandoned.

BACKGROUND OF THE INVENTION Semiconductor devices are sensitive to moisture and are therefore encapsulated directly in low melting glass or plastic materials or are mounted within a hermetically sealed metal case. In one particular application of the latter, a portion of the case is made of nickel-clad steel wherein the steel portion forms the inside of the case facing the transistor or other semiconductor device while the nickel forms the outer surface. The nickel metal is chosen for its resistance to corrosion. However, for ease of assembly, the clad material is formed with a flange which then is conveniently bonded, as by welding, to the metallic base which carries the semiconductor device. While this construction is economically desirable, unfortunately the construction results in exposure, to the atmosphere,.of the edge of the steel layer. Exposure of the steel to the ambient conditions results in corrosion which in turn can lead to rupture of the hermetic seal and eventual destruction of the semiconductor device within the casing.

It has been proposed to paint the case to inhibit corrosion. However, the use of paint is an added expense without commensurate benefit because chipping of the paint will expose the steel edge and allow corrosion to commence. Furthermore, specifications for the semiconductor devices, in some applications, require the outside casing to pass a solderability test; The use of paint on the exterior surface of the semiconductor case would be unsatisfactory for such applications.

Therefore an object of the invention is to provide a system to protect the exposed ferrous metal portions of the metal case of a semiconductor device from corrosion. It is another object of the invention to protect the ferrous metal portions of the metal case of the semiconductor device from corrosion without inhibiting the solderability of the metal case. Other objects of the invention will be apparent from the description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section view of a typical metalencased semiconductor device.

FIG. 2 is a flow sheet of the process of the invention.

DETAILED DESCRIPTION Referring now to FIG. I a metal-encased semiconductor device is generally illustrated at 2. The device comprises a base which may be copper, kovar or any other plateable metal or alloy of metals. Preferably the base 10 is plated with a layer 12 of noble metal such as gold. A semiconductor device is centrally mounted on one side of the base. Leads 22, 24 and 26 electrically contact appropriate portions of the semiconductor material and pass through the base via insulated bushings or sleeves 28. Metal cap 30 comprises a clad metal member having an iron or steel layer 32 and a nickel layer 34 clad thereto. Cap 30 is generally cylindrical having a closed end portion 40, a sidewall 44, and an outwardly flanged portion 50. Cap 30 is formed conveniently by dishing out a flat metal sheet. As can be seen from the drawing, flange 50 on cap 30 is attached to base 10. This attachment is made by welding or other suitable bond to provide an air-tight seal of cap 30 to base 10. However, this construction results in exposure of the end portion 32a of the iron or steel layer 32 at the edge of flange 50. This exposed portion of the iron or steel must be protected from the corrosive effects of the surrounding environment in order to maintain the air tight seal which protects the semiconductor material within the device.

In accordance with the invention, the semiconductor device is first assembled by suitably bonding flange 50 on cap 30 to base 10 having the semiconductor device already mounted thereto. Following the bonding of cap 30 to base 10 the outer surfaces of the device are treated to inhibit the corrosion of the exposed ferrous metal portions of the case.

The bonded metal case of the device is first carefully cleaned in appropriate cleaners-to remove all grease, salts, and oxides. For example, grease is removed by placing the case in a trichloroethylene vapor degreaser for about 5 minutes. After removal and drying, the case is washed in a detergent solution to remove salts and then washed in deionized water. The case is then immersed in a mild acid bath to remove oxides.

The cleaned case is then treated in an aqueous phosphate solution to phosphatize the exposed ferrous metal portions of the metal case. In a preferred embodiment, the metal case is immersed for about l020 minutes in a phosphate bath maintained at about 6080C preferably having the following composition in parts by weight:

' 2-3 parts 85 percent orthophosphoric acid 3-4 parts zinc phosphate 5-6 parts zinc nitrate balance water i After the metal case has been phosphatized, the case is treated with a compound which renders the phosphatized ferrous metal hydrophobic and provides an additional, transparent, protective coating over the ferrous metal. The compound must be a solid at the operating temperatures of the device (upto about -l00C) and preferably should possess good fluxing properties. In accordance with the invention, the coating compound is selected from the group consisting of a solid, organic acid, a combination of a solid organic acid and metal salt of a solid organic acid, and a solid ester of an organic acid and a long chain alcohol.

Examples of solid organic acids useful in the invention include stearic acid and other long chain organic acids having the general formula wherein n is at least 16.

Examples of metal salts of solid organic acids include specifically aluminum stearate and zinc stearate and, in general, compounds having the formula 0 M-oli om),.om

wherein n is at least 14 and M is a metal selected from Groups IIB and IIIA of the periodic chart found in the 43rd Edition of the Handbook of Chemistry and Physics,

published by the Chemical Rubber Publishing Company in 1961 on pages'448-449. 7

Examples of solid esters of organic acids and long chain alcohols include specifically waxes such as carnauba wax (myricyl cerotate), beeswax (myricyl palmitate), and spermaceti (cetyl palmitate) and, in general, esters having the formula where n is at least 15.

The compound is conveniently dissolved in a solvent to aid in its application as a coating to the metal case. Examples of such solvents include toluene, and xylene. The use of a halogenated solvent such as trichloroethylene in combination with such solvents as toluene is preferred to reduce the flammability of the coating solution.

When the solid organic acid or the ester is used, about 1-5 percent (by weight of the total solution) is used. However, higher concentrations can be used if soluble. When the solid organic acid is used with the metal salt about l-5 percent of the solid organic acid is used together with about 1 percent by weight of the metal salt. 7

In a preferred embodiment, a solution comprising stearic acid and aluminum stearate dissolved in a solvent comprising trichloroethylene and toluene is used to treat the phosphatized ferrous metal. Esters and solid organic acids such as, for example, stearic acid are preferred coating materials because of the excellent fluxing properties of such materials as well as their transparency whereby their presence on the casing is not objectionable to the user. The coating materials provide a corrosion resistant coating with a melting point of above 125C which is well above the operating temperature range of the device. Thus the coating will not melt and run off during operation of the device.

Combination of the solid organic acid with the metal salt of an organic acid provides a mixture having a still higher melting point thus making the mixture a hardermaterial which is even less likely to run out or become soft at the operating temperatures of the device. Thus when stearic acid is used, it is preferred to use it in combination with aluminum stearate. Alternatively, salts of other metals of Groups IIB and IIIA could be used such as, for example, zinc stearate. Stearic acid however is a better fluxing agent than the aluminum stearate. Both materials are, however, hydrophobic and are easily mixed together.

The amount of the stearic acid may vary from a little less than 0.5 percent by weight to as high as about 5 percent weight of the entire solution. Higher amounts are difficult to dissolve in the solvent system preferred. If other solvents are used having a higher solubility for stearic acid and its metal salts then higher amounts can be used. Perferably about 1 to 2 percent by weight of a metal salt of stearic acid such as aluminum stearate is used.

In the preferred embodiment the solvent comprises about 1-2 parts by weight trichloroethylene to 1 part by weight toluene. The toluene is a better solvent for the stearic acid and the aluminum stearate. Trichloro ethylene, however, is used to depress the flammability of the mixture. The amount of trichloroethylene used with the toluene therefore is an amount sufiicient to decrease the inflammability yet low enough to allow from 1 to 5 percent by weight of stearic acid and aluminum stearate to be dissolved in the solvent.

The phosphatized iron or steel has a very fine porosity surface which traps air molecules therein and therefore tends to inhibit the complete wetting of the metal surface by the stearic solution. In a preferred embodiment, therefore, the device is heated to about 100l50C to drive off the air molecules in the pores of the iron or steel. The casing is then quenched by dipping it into the stearic acid-aluminum stearate solution which is maintained at about room temperature. The casing remains in the coating solution preferably for about l-2 minutes. The sudden cooling of the casing which in turn compresses the expanded gases in the pores of the metal actually acts as a pump to draw the solution into the pores of the metal thus providing a more complete coating. Other coating methods, of course, could also be used such as, for example, a vacuum type impregnation. However, withthe particular solvent system used in the preferred embodiment, heating of the metal cap is preferable because of the low boiling point of the solvents.

In a specific example of the invention, a nickel-clad steel cap was welded to the gold plated base of a transistor to hermetically seal the transistor. The device was then placed in a trichloroethylene degreaser for 5 minutes, then removed and dried. The device was then washed for 2 minutes in a mild detergent solution and then rinsed off in deionized water. The device was then immersed in a 5 percent sulfuric acid bath at room temperature for 30 seconds and then rinsed in deionized water for 15 seconds.

The cleaned device was then immersed for 15 minutes in a phosphatizing solution maintained at C comprising:

2.17 parts by weight ortho phosphoric acid 3.78 parts by weight zinc phosphate 5.57 parts by weight zinc nitrate 88.48 parts by weight water 100.00 total The device was removed from the phosphatizing bath and, without rinsing, placed in an oven and heated to about ll0l25C. The device was then removed and immediately quenched in a room temperature solution comprising:

1.7 parts by weight stearic acid 0.8 parts by weight aluminum stearate 36.3 parts by weight toluene 61.2 parts by weight trichloroethylene 100.0

The device was kept in this solution for about l-2 minutes and then removed. The excess solution was allowed to drain off and then the device was air dried. After drying, the device was placed in an oven and heated to 200C for 5 minutes and then removed and cooled to room temperature.

The process was repeated using 1,000 devices. After treatment, each device was tested to determine its solderability in accordance with appropriate testing procedures. All of the devices passed this test. In addition, of the devices were subjected to salt mist test for 24 hours and then removed and examined. The corrosion resistance of the devices was found to be excellent with no visible traces of rust.

Thus the invention provides a protective system for the metal casings of semiconductor devices wherein exposed ferrous metal portions are phosphatized and then coated with a transparent, hydrophobic material which acts as a good fluxing agent to assist in the solderability of the metal case. The system therefore inhibits corrosion of the exposed ferrous metal portions of the metal case and thus preserves the hermetic encapsulation of the semiconductor device.

What is claimed is:

1. A semiconductor device, comprising:

a base a dished cap formed of a clad metal member having a ferrous inside layer and a nickel outside layer, said dished cap including an outwardly flanged portion with an edge portion, said cap being hermetically sealed to said base member, and said edge portion extending on the exterior of said sealed semiconductor device,

a phosphatizing coating extending over at least the ferrous portion of said edge portion,and providing a phosphatized surface thereon and a hydrophobic, transparent, solid organic solder flux means for enhancing solderability, having a melting temperature above 70C extending over said phosphatized surface. v

,2. The semiconductor device of claim 1 wherein said organic flux agent is selected from the class consisting of:

a. a solid organic acid having the formula f) CH5 (CHzhi'l- OH wherein n is at least 16;

b. a combination of a solid organic acid having the formula O CHAOHQhHl-OH wherein n is at least 16 and a metal salt of an organic acid having the formula 0 M-O-Hl (CHz) nCHa wherein n is at least 14 and M is a metal selected from Groups IIB and IIIA of the periodic chart; and

c. a solid ester of an organic acid and a long chain alcohol having the formula wherein n is at least 15.

3. The semiconductor device of claim 2 wherein said organic flux agent is dissolved in an organic solvent having flammability inhibiting properties.

4. The semiconductor device of claim 3 wherein said organic solvent consists of l-2 parts by weight trichloroethylene to 1 part by weight toluene.

5. The semiconductor device of claim 1 wherein said organic flux agent comprises a mixture of stearic acid and a metal salt of stearic acid selected from the class of Group B and Group IllA metals in the periodic chart.

6. The semiconductor device of claim 5 wherein said organic flux agent comprises l-5 percent by weight stearic acid and about 1 percent by weight of a metal salt of stearic acid selected from the class consisting of aluminum stearate and zinc stearate dissolved in a solvent comprising a mixture of 12 parts by weight trichloroethylene and 1 part by weight toluene. 

2. The semiconductor device of claim 1 wherein said organic flux agent is selected from the class consisting of: a. a solid organic acid having the formula
 3. The semiconductor device of claim 2 wherein said organic flux agent is dissolved in an organic solvent having flammability inhibiting properties.
 4. The semiconductor device of claim 3 wherein said organic solvent consists of 1-2 parts by weight trichloroethylene to 1 part by weight toluene.
 5. The semiconductor device of claim 1 wherein said organic flux agent comprises a mixture of stearic acid and a metal salt of stearic acid selected from the class of Group IIB and Group IIIA metals in the periodic chart.
 6. The semiconductor device of claim 5 wherein said organic flux agent comprises 1-5 percent by weight stearic acid and about 1 percent by weight of a metal salt of stearic acid selected from the class consisting of aluminum stearate and zinc stearate dissolved in a solvent comprising a mixture of 1-2 parts by weight trichloroethylene and 1 part by weight toluene. 